1. Field of the Invention
This invention relates to functional electrodeposited chromium layers having advantageous performance properties, and to a chromium plating bath and method for forming such chromium electrodeposits at high efficiencies.
2. Description of the Prior Art
Hexavalent chromium plating baths are described in U.S. Pat. Nos. 3,654,101 to Aoun, and 3,745,097, 4,450,050, 4,472,249 and 4,588,481 to Chessin et al. These baths generally are intended for decorative chromium plating or for hard, functional, chromium electrodeposition. Decorative chromium plating baths are concerned with deposition over a wide plating range, so that articles of irregular shape can be completely covered; functional chromium plating, on the other hand, is generally designed for regularly shaped articles, where plating at higher current efficiency and at higher current densities is important.
Functional hexavalent chromium-plating baths containing chromic acid and sulfate as a catalyst generally permit the deposition of chromium on a basis-metal substrate at cathode efficiencies of about 12% to 16% at current densities of from about 15 to about 95 amperes per square decimeter (asd), current efficiency being defined as the ratio of the amount of current used to achieve plating to the amount of current applied to the bath. Mixed-catalyst chromic-acid plating baths containing both sulfate and fluoride ions generally allow chromium plating at higher cathode efficiencies, e.g., from 22 to over 26%. However, the presence of fluoride ion in such baths causes etching of the generally ferrous-based metal substrate.
Other chromium plating baths which use iodide, bromide or chloride ions as additives can operate at even higher current efficiencies, but such baths produce chromium deposits which do not adhere well to the substrate, and which are dull in appearance, or at best only semi-bright. For example, Chessin, in U.S. Pat. No. 4,472,249, describes a high-energy-efficient functional chromium electroplating bath which operates at very high current efficiencies, e.g., about 50%. These baths generally consist of chromic acid, sulfate ion, iodide and a carboxylate, and are used at conventional current densities between about 15 and about 95 asd. Unfortunately, this bath has adherence problems and poor low-current-density etching, and provides only a semi-bright deposit.
Chessin and Newby, in U.S. Pat. No. 4,588,481, describe a method for producing non-iridescent, adherent, bright chromium deposits at high current efficiencies, without low-current-density etching. This method involves plating at a temperature of from about 45 to about 70 degrees Centigrade (.degree.C.) from a functional chromium plating bath consisting essentially of chromic acid and sulfate ion, and a non-substituted alkyl sulfonic acid having a ratio of sulfur to carbon of more than 1/3, the bath being substantially free of carboxylic or dicarboxylic acid.
Suzuki and Tsukakoshi, in U.S. Pat. No. 4,543,172 and 4,592,819, describe a very-high-speed apparatus for electroplating metals, e.g., chromium, within a very short time period. In this method, a flowing plating liquid is circulated at a high speed between a workpiece and an anode in the plating chamber. The operating current densities permissible in such a system can range from 775 to 1400 asd, which is an extraordinarily high current density, but which enables plating to occur very rapidly. In fact, the apparatus is referred to in the art as a "Rapid Plating System" (RPS). Unfortunately, the demands of this system necessitate a chromium-plating bath which can operate under the extreme RPS conditions in order to provide higher-performance chromium electrodeposits.
Accordingly, it is an object of the present invention to provide a high-performance electrodeposited chromium layer, a chromium-plating bath, and a method for forming such chromium electrodeposits, particularly under RPS conditions.
A specific object herein is to provide chromium electrodeposits which are adherent, bright, smooth, hard, wear resistant, exhibit a low coefficient of friction, and which can be formed at high efficiencies and at useful current densities, including both the very high operating-current densities of rapid plating systems, and the low current densities of conventional chromium plating.